In modem manufacturing environments, vision systems are often used to test products, components, or sub-assemblies in order to verify that the item has been properly manufactured or assembled. A vision system performs, in an automated way, a visual inspection analogous to that which would otherwise be done by a person. Among other benefits, the vision system can relieve a human operator of the tedium associated with such an inspection, thus freeing him to perform other functions, and can automatically maintain quality records and analyze trends so as alert the manufacturing personnel of actual or potential quality problems with the manufactured item.
Vision systems typically utilize a video camera to capture and digitize images of the item being tested, and a computer connected to the video camera to analyze the digitized images and compare them to what is expected to be observed. For the vision system to operate properly—that is, to correctly identify tested items as being either good or bad—each item being tested must be appropriately illuminated for the vision test being performed. If items are illuminated improperly, the vision system may incorrectly identify a good item as a bad one, and reject it. Incorrectly rejecting a significant number of good items results in increased manufacturing costs which ultimately get passed on to the consumer.
In order to minimize the number of items which are incorrectly rejected by the vision system as non-complying, once the appropriate level of illumination for a particular test is determined, the light source should be capable of providing a certain constant light output (or intensity) for desired intervals over a long period of time. Furthermore, if a varying amount of ambient light can also illuminate the item during testing, the light source should be capable of both detecting this ambient illumination and varying its light output so as to maintain the total illumination on the item at a constant level.
Certain vision applications require the light source to provide light of different intensities during different tests, or for testing different items. For example, two identical items supplied from different vendors or produced from two different manufacturing runs may have different reflectivity, requiring different illumination levels for the vision system to operate properly. Or, in one test the vision system may view the item through a protective wrapping that requires more illumination, and in another test without the wrapping that requires less illumination.
Other vision applications require the light source to provide different color lighting. For example, a first color light may be used to view surface features on a partially-transparent window of an item, while a second color light is used to view the internal features of the item through the window.
In addition, to view surface features of an item that project in a direction towards the video camera lens, it is advantageous for the light source to provide side, rather than frontal, illumination. Providing side illumination to these surface features can generate shadows which are more easily detected by the vision system. Also, when illuminating highly reflective items, side illumination may reduce certain reflections which can interfere with proper vision system operation.
Some previous light sources use halogen or incandescent lighting, which often are subject to fading over time. In addition, such light sources tend to have relatively short lifetimes, requiring frequent replacement of the light sources. These light sources also tend to generate excessive heat which often requires cooling so as not to affect the item being tested or other elements of the vision system, thus resulting in added cost or complexity of the light source. While fluorescent light sources tend not to produce excessive heat, fading over time is still a problem. With any of the above light sources, filters are required in order to provide different color light output. Such filters are expensive, not easily changed, and provide only a limited number of discrete light colors. Other previous vision system light sources use light-emitting diodes of a certain color. Where the intensity of any of these light sources is selectable, it is typically done manually.
Accordingly, the need still exists for a light source that can produce a specified light output of a specified color, can maintain a constant light output for desired intervals over a long period of time, and can automatically adjust the light output so as to maintain a constant illumination level on an object under a variety of ambient lighting situations.